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Its no point beating yourself up over this situation where you have been let down by others, its a lesson for you to learn in terms of how you will handle money to be paid to others in the future, and that is to be cautious always.
Your friend is no doubt aware of your situation and as much as you want to pay back a lump sum - sometimes even paying back small installments at a $100 or $50 at a time goes towards the goodwill and intentions you have.
When you are in the position to increase the payments than do so, but I would think that until then, something is better than nothing and your friend should appreciate that - after all, they have learned a valuable lesson as well to not mix money and friendship.
I trust this information is of assistance and that you will kindly accept my answer as I do wish you the best in this matter